1. Field of the Invention
This invention relates generally to a vehicle powertrain having an input clutch. More particularly, the invention pertains to a method for controlling the clutch during a vehicle launch condition.
2. Description of the Prior Art
A vehicle launch is a low-speed drive-away event in which a vehicle with a transmission is accelerated in response to the vehicle operator depressing an accelerator pedal. When the vehicle powertrain is equipped with a conventional automatic transmission, launch behavior is largely regulated by the torque converter, which ultimately sets engine speed and wheel torque as a dynamic function of engine torque. In this conventional powertrain setting, the torque converter characteristics are first engineered to trade off fuel economy and driving performance, then engine pedal maps (the engine control software relationship between accelerator pedal position and engine torque) are tuned to provide the best vehicle feel for the given converter design.
Launch control is used to facilitate vehicle motion from standstill after the driver presses the gas pedal indicating his intention to initiate vehicle motion. In automatic transmissions the launch functionality is provided with a torque converter, which has inherent positive characteristics, such as torque multiplication at low speed ratio, characteristic of launch situations both on level roads and hills. A torque converter also provides hill holding capability. It provides a natural damping or filtering of any possible drivability and NVH-related disturbances that may occur due to jerky gas pedal motion caused by a driver's change-of-mind and similar actions. A torque converter, however, dissipates energy, reducing fuel economy, and imposes a space and weight penalty.
Due to these limitations, an emerging trend in automatic transmission design eliminates the torque converter and replaces it with appropriately controlled friction clutches. One such example is the so-called Powershift (PS) transmission, consisting of only two clutches (one for odd and one for even-numbered gears), synchronizers-like couplers, manual transmission-like gears, sensors and either electrical or electro-hydraulic actuation of the clutches and synchronizers.
Such transmissions offers a number of advantages including low parts count, simplicity, robustness of design, and substantial commonality with manual transmissions, thereby facilitating common production.
The Powershift transmission enables the use of algorithms that can control the clutch in ways more complex than the passive torque converter, eliminating or reducing some of the design tradeoffs inherent in the passive torque converter system. For example, the clutch can be fully opened when the driver presses the brake pedal, eliminating transmission drag on the idling engine, thereby improving fuel economy.
In the automatic clutch setting, the intent of the launch function is to provide a wheel torque that reflects driver demand from the accelerator pedal and provides an engine speed trajectory that meets the competing requirements of providing a “power-on” feel and sound to the driver while limiting clutch component wear associated with slip across the device.
A need exists in the industry for control of the input clutch such that robust, optimized performance is obtained.